1. Field of the Invention
The present invention relates to an image obtainment method apparatus for obtaining an image of a region to be observed by illuminating the region to be observed with illumination light output from a light source, and by imaging, by an imaging device, light output from the region to be observed by illumination with the illumination light. In particular, the present invention relates to control of the light amount of illumination light, and control of the electronic shutter speed of the imaging device.
2. Description of the Related Art
Conventionally, endoscope systems for observing tissue in a patient's body are widely known. Further, electronic endoscope systems that obtain a visible image of a region to be observed in a patient's body by imaging the region to be observed by an imaging device, and that display the obtained image on a monitor screen are widely used.
Such endoscope systems perform short-distance imaging, long-distance imaging, and the like. In short-distance imaging, a leading end of an insertion unit of an endoscope system, which is inserted into a patient's body, is placed close to a lesion or the like to image the lesion or the like. In long-distance imaging, a relatively wide range including a lesion and the vicinity thereof is imaged.
For example, when the light amount of illumination with which a region to be observed is illuminated is constant, relatively bright image signals are obtained in short-distance imaging, because a distance between the lending end of the insertion unit and the lesion is short. However, in long-distance imaging, relatively dark image signals are obtained, for example, when the inside of an intestine is imaged, because the target of imaging is tubular. Therefore, there is a problem that visible images displayed on a monitor are too light or too dark in some cases.
Therefore, a method for automatically changing the light amount of illumination light based on the luminance component or the like of image signals obtained by an imaging device so that the brightness (luminance) of visible images displayed on the monitor becomes constant has been proposed. Specifically, the light amount of illumination light is automatically reduced, as the luminance component of image signals obtained by the imaging device is larger, and the light amount of illumination light is automatically increased, as the luminance component of image signals obtained by the imaging device is smaller.
However, the light amount of illumination light output from a light source is limited, and the variation range of the aperture of a diaphragm for adjusting the light amount of illumination light output from the light source is limited, because of the mechanical precision and accuracy of an apparatus, or the like. Therefore, the brightness of visible images is not maintained at a constant level in some cases.
Meanwhile, in the aforementioned endoscope systems, dynamic image (video) imaging is performed by performing real-time imaging by the imaging device. Further, a still image is obtainable by obtaining a desirable frame from a series of frames of visible images obtained by dynamic image imaging.
Since the still image is obtained especially to perform precise observation, it is desirable that a blur in the still image is minimized. Therefore, it is desirable that the electronic shutter speed of the imaging device is as high as possible.
For example, U.S. Pat. No. 6,078,353 (Patent Document 1) proposes a method in which sufficiently bright visible images are obtained by setting a low electronic shutter speed of the imaging device when the aperture of a light source is fully open, in other words, when a region to be observed is dark. In contrast, when the aperture of the light source is not fully open, in other words, when the region to be observed is light, a high electronic shutter speed is set.
However, Patent Document 1 proposes switching the electronic shutter speed only to three levels, namely, 1/60 second, 1/200 second, and 1/400 second, depending on whether the aperture of the light source is fully open or not. If the electronic shutter speed is switched only to these three levels, it is still impossible to maintain a constant brightness of visible images in some cases. In such cases, unpleasant visible images are provided for observers, and there is a risk of imposing a burden on the observers.
Further, in Patent Document 1, after the electronic shutter speed is set to one of the three speeds, the brightness is adjusted mainly by changing the aperture of the light source. However, for example, when the frame rate of the imaging device is high, and the light amount needs to be adjusted at high speed by changing the aperture, a highly accurate mechanism that can provide high response speed and highly accurate adjustment of the position of the aperture is necessary. Therefore, the mechanism for controlling the light amount becomes complicated, and that causes an increase in the cost of the system.